1. Field of the Invention
The present invention relates to a refractive index distribution type lens (hereinafter referred to as a graded index lens) in which the refractive index changes along the radial direction for the rod lens. More particularly, the invention relates to a graded index lens obtained by treating a raw glass material by ion exchange using silver to form a refractive index distribution in the radial direction.
2. Description of the Related Art
A refractive index distribution type optical element in which the refractive index parabolically changes in a section thereof from the center along the radii has the same image-forming function as spherical lenses even when both sides thereof are flat. Since this type of optical element has advantages, for example, that a lens having a small diameter and a short focal distance can be easily produced, it is extensively used in optical heads for copiers, printers, facsimile telegraphs, and the like and in other applications. Such refractive index distribution type optical elements include graded index lenses and refractive index distribution type fibers.
The glasses of the related art which are produced through drawing and a subsequent step of ion exchange conducted by immersion in molten salts and are for use as refractive index distribution type optical elements (e.g., lenses) include the following three main kinds: (1) thallium-containing glasses, (2) cesium-containing glasses, and (3) lithium-containing glasses.
The thallium-containing glasses can give graded index lenses having an extremely large angular aperture because they have high electronic polarizability. However, these lenses have enhanced chromatic aberration and, hence, cannot be used especially in optical systems dealing with color images.
The cesium-containing glasses have reduced chromatic aberration unlike the thallium-containing glasses. However, since it is impossible to incorporate a large amount of cesium into a glass, the lenses obtained from the cesium-containing glasses are limited to those having a small angular aperture. In addition, these glasses have a drawback that they have an exceedingly high melting temperature.
The lithium-containing glasses have reduced chromatic aberration and a moderate melting temperature and are hence used extensively. However, since glasses containing a large amount of lithium ions are highly apt to devitrify, a graded index lens having a large angular aperture cannot be produced therefrom.
Lenses having a large angular aperture are therefore produced from the thallium-containing glasses. However, since thallium is toxic, production of the glasses containing a large amount of thallium and use of a molten salt containing a large amount of thallium are undesirable from the standpoint of environmental pollution.
Besides the ions shown above, silver ions are thought to contribute to the attainment of a large difference in refractive index as shown in JP-A-61-261238 and JP-A-62-100451. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.) Silver ions are advantageous from the standpoint of producing a lens dealing with color images because use of silver results in reduced chromatic aberration as compared with the case of using thallium. Since silver ions are generally apt to become colloidal, the references cited above propose a glass composition containing a large amount of a phosphorus component in order to inhibit silver ions from forming a colloid.
However, such glasses containing a large amount of a phosphorus component have poor weatherability and insufficient suitability for practical use. The glasses containing a large amount of a phosphorus component further have problems that the glasses during ion exchange react with a nitrate to yield a devitrification product on the glass surface and that the glasses themselves partly dissolve in the molten salt. When a melt of a salt other than nitrates, e.g., a melt of a sulfate or halide, is used, there is a problem that since such a melt highly corrodes metals and glasses, an appropriate container for holding the molten salt is not easily available.
An improvement of the glass composition proposed in JP-A-62-100451 is the glass composition disclosed in JP-A-4-2629. However, this glass composition also is still insufficient in stability in molten salts and weatherability and is hence unsuitable for practical use.
On the other hand, an aluminosilicate glass is known as a glass composition which contains no phosphorus components and in which silver ions do not form a colloid. In general, incorporation of an alkali into a silicate glass cleaves the silicate framework to form nonbridging oxygen (hereinafter referred to as xe2x80x9cNBOxe2x80x9d) strongly bonded to alkali ions. When a glass in which NBO is present is subjected to ion exchange to incorporate silver ions thereinto, then the silver ions incorporated are reduced by the NBO to form a silver colloid and thereby color the glass. Consequently, such a glass cannot be used as a lens.
In contrast, when Al2O3 is added to a silicate glass, the Al2O3 is incorporated in the form of AlO4xe2x88x92 and bonded to an alkali. Because of this, the amount of NBO in the glass is reduced and silver ions are apt to be present stably in the ion form. Since AlO4xe2x88x92 bonds to an alkali ion in a proportion of 1:1, the amount of NBO in the glass is minimum (becomes zero in some glasses) when [M]/[Al] is 1 ([M] and [Al] represent the molar concentrations of the alkali ion and AlO4xe2x88x92, respectively, in the glass). Consequently, the glasses in which silver ions are contained most stably are the glasses in which [M]/[Al] is 1.
Incidently, for increasing the angular aperture of a lens having a radial distribution of refractive index, it is necessary to increase the radial difference in refractive index. Refractive index difference is almost proportional to the concentration of silver ions. In order for a glass to have an increased angular aperture, it should therefore contain a large amount of an alkali to be replaced by silver ions. For enabling silver ions to be stably present in an aluminosilicate glass, it is necessary to increase the concentration of Al2O3 as the alkali concentration in the glass is increased. However, glasses having an increased Al2O3 concentration have an elevated melting temperature and, hence, glass products of satisfactory quality (free from striae, bubbles, etc.) are difficult to produce therefrom. Although a technique of reducing the Al2O3 concentration in the glass may be used for lowering the melting temperature, this results in a reduced amount of silver ions capable of being contained without forming a colloid, making it impossible to obtain a large difference in refractive index.
It is known that incorporation of B2O3 into a glass is effective in lowering the melting temperature while inhibiting silver ions from forming a colloid (Glastech Ber., 64 [8] 199(1991); Appl. Opt., 31 [25] 5221(1992); J. Non-Cryst. Solids, 113 37(1989)). (In glasses, boron is trivalent like aluminum.) However, too high a concentration of B2O3 in a glass poses problems that the glass has reduced durability and the rate of ion exchange of silver ions is low. Consequently, the concentration of B2O3 which can be incorporated is limited.
In JP-A-4-219341 are given BeO, CaO, Ga2O3, La2O3, MgO, Nb2O3, Ta2O3, Yb2O3, ZnO, and ZrO2 as ingredients which can be used, besides B2O3, in place of Al2O3 while inhibiting silver ions from forming a colloid. However, there is no description therein concerning whether the replacement lowers the melting temperature or not.
As described above, none of the techniques of the related art has succeeded in providing a high-quality lens which has a radial distribution of refractive index with a large radial difference in refractive index and has fully satisfactory properties.
An object of the invention, which eliminates the problems of the techniques of the related art described above, is to provide a highly durable, excellent lens which has a radial distribution of refractive index required for optical designs and which has satisfactory quality, i.e., which is produced from a glass composition having a low melting temperature and a rate of ion exchange sufficient for practical use and enabling silver ions to be stably present in the glass.
The invention provides a graded index lens obtained by treating a raw glass material by ion exchange using silver to form in the raw glass material a refractive index distribution in the radial direction, the raw glass material being constituted of a glass composition comprising the following components:
15 less than Na2Oxe2x89xa630 mol %,
10 less than Al2O3xe2x89xa625 mol %,
27.5xe2x89xa6SiO2xe2x89xa655 mol %,
3xe2x89xa6B2O3xe2x89xa618 mol %,
2.5xe2x89xa6MgOxe2x89xa618 mol %,
0xe2x89xa6Ta2O5xe2x89xa65 mol %,
0xe2x89xa6La2O3xe2x89xa63 mol %,
0xe2x89xa6BaOxe2x89xa63 mol %, and
0xe2x89xa6ZrO2xe2x89xa63 mol %.
Namely, according to the invention, by incorporating MgO into an aluminoborosilicate glass, the melting temperature of the glass can be lowered while maintaining high durability and silver ions can be stably incorporated in the form of ions into the glass. As a result, a lens having a necessary radial distribution of refractive index is realized.
It is known that polyvalent ions generally serve to considerably reduce the rate of diffusion of monovalent ions. However, it has been reported that MgO and ZnO give polyvalent ions which relatively less reduce the rate of ion exchange (R. H. Doremus, xe2x80x9cGlass Science 2nd. ed.xe2x80x9d p.278, John Wiley and Sons) and that these polyvalent ions may diminish NBO in glasses (Appl. Opt., 31 [25] 5221(1992)).
However, experiments made by the present inventors revealed that when ZnO is incorporated into an aluminoborosilicate glass, phase separation occurs in the glass. Incorporation of MgO did not result in such phase separation at all.
The present inventors have found that the object of the invention can be accomplished by incorporating MgO into an aluminoborosilicate glass as a glass composition for use as a raw glass material from which a lens having a radial distribution of refractive index is to be produced. The invention has been thus completed.
In the invention, the glass composition is preferably as follows.
18xe2x89xa6Na2Oxe2x89xa630 mol %
15xe2x89xa6Al2O3xe2x89xa625 mol %
30xe2x89xa6SiO2xe2x89xa650 mol %
5xe2x89xa6B2O3xe2x89xa618 mol %
3xe2x89xa6MgOxe2x89xa615 mol %
0xe2x89xa6Ta2O5xe2x89xa63 mol %
0xe2x89xa6La2O3xe2x89xa63 mol %
0xe2x89xa6BaOxe2x89xa63 mol %
0xe2x89xa6ZrO2xe2x89xa63 mol %
The glass composition in the invention is more preferably as follows.
20xe2x89xa6Na2Oxe2x89xa630 mol %
15xe2x89xa6Al2O3xe2x89xa622.5 mol %
30xe2x89xa6SiO2xe2x89xa645 mol %
7xe2x89xa6B2O3xe2x89xa615 mol %
5xe2x89xa6MgOxe2x89xa612.5 mol %
0xe2x89xa6Ta2O5xe2x89xa63 mol %
0xe2x89xa6La2O3xe2x89xa63 mol %
0xe2x89xa6BaOxe2x89xa63 mol %
0xe2x89xa6ZrO2xe2x89xa63 mol %